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More than 20 homeowners claim roadworks on Auckland's Southern Motorway is causing hundreds of thousands of dollars damage to their houses.

Conifer Grove homeowner Gayleen Smith has an estimated $500,000 damage to her home with an engineering report commissioned by the NZ Transport Agency finding on "balance of probabilities" the house had been affected by the roadworks.

The NZTA - which is in charge of the $268 million project to add new lanes to the motorway - denies homes are being damaged.

Dianne and Cliff Walker say roadworks on the Southern Motorway have sent cracks snaking through the walls of their home. Photo / Diego Opatowski

Yet 22 homeowners have complained to the agency, while 25 have registered with a law firm considering a class action, saying vibrations from the works have sent cracks snaking through their walls and ceilings.

Thorn called on Prime Minister Jacinda Ardern to intervene in the stoush because homeowners had so far unsuccessfully been caught in a David-and-Goliath battle with NZTA and its contractor and lived through two years of hell.

Gayleen Smith is one of 25 Auckland home owners registered with a law firm considering class action over damages they claim was caused by roadworks on the Southern Motorway. Photo / Diego Opatowski

A list complied by her firm showed 15 homeowners had reported cracks in internal and external walls, as well as some with sunken floors and damaged windows, concrete, tiles and pipes.

It raised mental health issues and had led some to take medication, she said.

"I talk to a lot of victims and I hear a lot about people not being able to sleep, but I've never heard of something on this scale," Thorn said.

"It seems every single person there has got a sleeping disorder."

The Government needed to buy out the homes of those affected or make settlements as soon as possible, she said.

The Herald understands some smaller claims have been settled and that NZTA has agreed to "relevel" one home in Manurewa East, near Dutt's, after the owner complained her foundations had sunk because of the roadworks.

NZTA senior manager project delivery Chris Hunt said the agency was working with the homeowner to agree on repairs "without accepting liability" for the damage.

He also said that - despite an initial report into Smith's Conifer Grove home concluding it had likely been affected by vibrations or groundwater movement from the roadworks - NZTA wanted to conduct further investigations.

"We acknowledge the opinions given in the report but consider it not conclusive in respect of causation or liability," the agency said in a letter to Smith.

Hunt also said the agency had done its best to be a good neighbour to those near the roadworks and had obeyed strict resource consent conditions and monitored vibration levels to ensure they were within guidelines.

But Smith feared the longer NZTA delayed, the greater the damage would be to her home as extra water seeped into the cracks in its exterior walls.

Fellow Conifer Grove resident Dianne Walker also fears her home is being further damaged by water seeping into exterior wall cracks.

She tried to claim the costs with her insurance company, which ordered her not to repair the cracks because it would void her claim.

Then after waiting nine months and receiving reports stating the damage had likely been caused by the roadworks, her insurance company finally told her it would not be paying up after all because vibration damage was not covered by the policy.

"Why even bother with the reports if we didn't have vibration cover," Walker said.

"The whole thing wasted nine months of my time and now I have to go back to the NZTA [to seek damages]."

THE STORY SO FAR

* $238 million project underway to add extra lanes to Southern Motorway* 25 nearby homeowners say the roadworks have damaged their houses and registered with a law firm considering launching a class action* One Conifer Grove home owner has an estimated $500,000 in damages, while 14 others say roadworks have sent cracks snaking through the walls of their homes* NZTA denies the roadworks are behind the damage